Electrical signaling system



Jan. 14, 1947. a. M. HADFI TELD 2,4142% ELECTRICAL SIGNALING SYSTEM Filed Aug. 19, 1943 2 Sheets-Sheet 1 W L! Ll I 9 E vF I m.

LWI Lwl FIGQS. -F|G.6. INVENTORY BERI'RAM MORTON HADFIELD ATTORNEY Jan. 14, 194? B. M. HADFIELD 2,414,297 ELECTRICAL SIGNALING SYSTEM Filed Aug. 19, 1943 2 Sheets-Sheet 2' INVENTOR BERTRAM momou HADFIELD ATTORNEY Patented Jan. 14, 1947 Zeiliit? ELECTREQAL SEGNALING SYSTEM Bertram dorton Hadfield, Harrow Wcald, England, assignor to Automatic Electric Laboratories inc, Chicago, iii, acorporation of Deiaware - Application August 19, 1943, Serial N0. 499,163

In Great Britain September 4, 1942 6 Claims.

.The present invention concerns improvements in or relating to electric signaling systems and particularly to signal system in which the signaling currents are of a single current type.

The object of the invention is to convert a single current effect as received over a transmission medium into a double current effect at the reception apparatus.

The expression double current, effect as used herein is intended to mean that the receiving equipment is energised to substantially equal. extents in the positive and negative directions while the expression single current effect as used herein applies to signaling currents whose amplitudes are not equal in the positive and negative directions and applies to signaling currents which vary from Zero to some positive value or vice versa, which vary from a small positive to a large positive value or vice versa or which vary from a. positive to a substantially different negative value, it being understood of course that the terms positive and negative as used. in this latter expression may be interchanged.

According to the invention signal currents having a single current effect are adapted to produce of themselves a bias effect on the receiving equipment such that the receiving equipment is energised to substantially equal extents in the positive andnegative directions.

According to one feature of the invention the signaling current has initially a value other than zero and signals are subsequently transmitted by reducing the value of the signaling current to zero, a bias effect being derived by the reception apparatus from the initial signaling current, said bias having a value proportional to s: id value.

According to another feature of the invention the bias effect is designed so as to have a magnitude substantially equal to the mean of two values of .the signaling currents such that it would cause the two vaiuesof thesignalingcurrent to exert an equal and opposite operation of the reception equipment. In the case of one value of the signaling current being zero, the bias effect would have a magnitude substantially equal .to that .of the effect .01" one half the other value of the sig naling current such that it wouldcause an equal and opposite operation of the reception equipment if present in the ,absenceof said latter value of the signaling current.

The bias effect so produced may bepresent at the same time the signaling current so that the net operating magnitude is one third of that which would otherwise be obtained or the bias effect may only become effective when the value of the signaling current is zero. In either case the object is to produce equal and opposite operating effects on the transmission equipment, such as a relay, respectively during and after the transmission of the signaling current.

According to another feature of the invention the bias effect so obtained is made of substantially constant magnitude throughout the period when the value of the signaling current is zero by designing the bias circuit to have a delay time function which produces a negligibl change in the bias effect over the period of the longest signal, i. e., the longest period during which the value of the signalin current is zero.

According to another feature of the invention the bias effect may be produced in a rapid manner upon transmission of the initial signaling current by making the time function of the bias circuit non-linear so that the attainment of the bias effect is very much faster than the decay.

It is well-known that one of the advantages possessed by double current effect transmission and reception, is the reduction of received signal distortion obtainable by causing the reception apparatus to operate and release at or about the half total amplitude points on the received signal waveform with time. This effect is of course achieved by making these points of zero amplitude and using a balanced or neutral reception apparatus of great inherent sensitivity. The performance of such a system is thereby made to a large de ree independent of signal amplitude, whereas a single current system is largely dependent on signal amplitude. From the above features it will be seen that the invention is concerned with producing this typical double current effect at the reception apparatus, although the transmission consists only of a single current effect.

While the double current effect produces a minimum of distortion with changes in received signal level, the absolute distortion will depend on the degree to which the received signal waveforms with time are symmetrical with respect to one another. This depends on the linearity of the transmitting arrangements and medium and as such Will affect the normal double current method and the invention to the same degree. It will therefore be assumed that the transmitting arrangements are substantially linear so that the best results may be obtained.

As regards non-linear effects in the reception apparatus comprising the invention, the same general restrictions as apply to th normal double current method must be met, namely that the responsive apparatus must be of balanced or neutral type and of high sensitivity, and the response over equal positive and negative inputs sufficient to cause operation must be substantially linear. The ultimate responsive apparatus of the inven:- tion will therefore consist of a balanced or neutral device such as a polarised relay, and the intermediate apparatus subsequent to the conversion of the received single current effect into a double current efiect will also be linear and equally responsive to positive and negative inputs sufficient to cause operation of the relay.

The invention will be better understood by referring to the accompanying drawings in which:

Fig. 1 shows the application of the invention to a simple relay.

Fig. 2 shows how the arrangement of Fig. 1 may be applied to a speech transmission bridge as used in automatic telephone systems.

Fig. 3 shows the application of the invention to a valve energised relay, while Fig. 4 shows an alternative to that shown in Fig. 3.

Fig. 5 shows the arrangement of Fig. 5 applied to a telephone signaling system.

Fig. 6 shows a modification of the arrangement shown in Fig. 5, to avoid the effect of earth currents.

Fig. '7 shows the application of the invention to cause a device to operate continuously during a train of impulses, while Fig. 8 shows a portion of the circuit of a final selector in which the B and C relays are controlled difierently in accordance with the principles of the invention.

Referring to the drawings, Fig. 1 shows a schematic diagram of the polarised relay, comprising opposing windings LI and L2, the latter having one half the number of turns of the former and constituting the bias winding. The bias winding and a large series inductance L3 are shunted by condenser C, a resistance r symbolising the total mesh resistance. The received current 2'! produces an operating flux in LI, and a biassing flux of one half this magnitude in L2, in the. condition prior to signaling, 1. e. il equals i2. Upon cessation of il, constituting th signal, the bias flux is retained by i2, since L3 and C produce a long decay period for i2. The inductance L3 besides assisting this process, also permits the back E. M. F. produced on L2 by the cessation of ii in L! (i. e.. due to the mutual inductance) to function without material alteration, thus permitting the normal operatin characteristics of the relay. Therefore to the total change of signal effect there is added a bias effect exactly equal to one half the change which persists for the duration of the signal. Hence the relay operation corresponds to that which it would have had if the transmission had been by double current.

.here are two major. difliculties in the design of the circuit: firstly, the required permanence of the bias current over the lon est single cessation of signal and secondly, the linearity of the circuit which involves a gradual reduction in the bias current over a period of prolonged signa ing. As regards the first it is found'that very large values are necessary for L3 and C, if for instance a 10% drop in bias current is permitted. and with regard to the second. this means that no matter how lar e L3 and C are made. the ultimate bias current i2 must become equal to the m ke percentage fraction (or average value) of the input signal when the latter is repeated indefinitely. If the bias circuit could be made non-linear and have a short buildup time, the second difficulty would disappear and with it the corresponding practical difficulty that there may be very little time in which to attain the bias eifect before signaling. It is difficult to see how this may be done, since the energy to be stored rapidly in L3 and C must come from the input signal, but the same effect could be obtained if the bias winding were energised from a valve, and the necessary non-linear buildup and decay circuit placed in the grid circuit of the valve. This leads to the more preferred embodiments of the invention.

Fig. 2 shows how the simple relay circuit may be readily adapted for use with a well-known speech transmission bridge as used in automatic telephone systems. The windings of Li, L2 and L3 are split and equally divided between the two line wire connections to the speech transformer L4, L4; condenser K serving the usual purpose of bypassing the speech currents from the relay circuit. E represents the normal signaling battery.

The previous example showed a typical circuit in which the signal bias effect was efiective at all times. It is obvious that a valve circuit could be arranged to give the same bias eifect from the received signal current, but the arrangement would sufier from the difficulty of keeping the signal and bias eifects at a constant ratio with variations of the valve, either initial variation or variations as the Valve aged. In the circuit to be described it is arranged that the conversion is accomplished prior to the valve stage. In this manner the required conversion action is consistently maintained irrespective of such variations and more particularly of amplitude variations, which permits the subsequent valve stage to have overload values without affecting the operating of the relay, provided the latter is always effected within the substantially linear range of the valve stage.

Fig. 3 shows the basic circuit in which the received current ii, of symmetrical waveform with time, passesthrough two nominally equal resistances ri, 1'2. (The case where the received waveform is non-symmetrical with time is dealt with later.) Resistance M is shunted by the circuit comprising condenser C, resistance R and rectifier MR. the latter being conducting to the charge current of C. The voltage across 1'2, plus R which is of the desired doubl current form, is applied to the grid and return cathode circuit of the valve V via the grid resistance R9. The cathode resistance Rc serves to bias the valve to the amplifying state and to produce negative feedback for the purpose of linearising the valve stage in well known manner. The operating winding of the polarised relay P is connected in the anode circuit and a further winding serves to neutralise the ampere-turns due to the steady anode currents by means of a current from the battery E controlled by resistance Rn.

The two equal resistances r! nd T2 are of value not greater than will produce a voltage on each suificient to operate the relay via the valve, and are connected in series so that the single direct current passes therethrough and produces the aforesaid voltage. The value of R is much greater than either of the two former resistances, say at least times. When the single current efiect is applied the condenser C charges Via MR up to the steady voltage on H, and when this has been accomplished the net grid voltage is due solely to that on R2. Upon cessation of the single current. effect to produce. the. first signal, the corn denser discharges via 1! and R and produces a grid. voltage of opposite sign to that formerls existing, and of substantially the same magnitude (1.. e., to if R is, 100 times 1|). Provided this voltage remains substantially unaltered for the duration of. the longest received cessation period the conversion to the double current effect has been achieved. This can here be arranged easily since only voltage. effects are required; for instance a. discharge time constant of 20 times the.

longest signal, will result in a 5% drop in this bias effect, whilst a factor of 59 will result in. a 2% drop, and, so, on. In a like manner to that described in the first embodiment the insertion of an appropriate inductancein series with R will considerably facilitate the choice of suitable discharge circuit values for prolongation of the bias effect.v

In the present case, however, the design of the discharge circuit need not be hampered by consideration of the time to build up the bias, since the charge time is reduced to the minimum by shunting R together with the inductance if used, with a rectifier, so that the latter is conducting when C charges and non-conducting when it discharges. The charge time constant will then be at the most times the discharge time constant.

The above represents only one method whereby the conversion of the input single current eifect to a double current effect may be achieved on a voltage basis; modifications thereof will doubtless occur to those skilled in the art. For instance a simple modification to the above described circuit will enable the postulated high ratio between R and r! to be reduced and enable resistances of commercial tolerances to beused without sacrificing the desired equality between the operating and bias effects. By making T! a variable resistance larger than is required, or converting 7'' and r2 into a potentiometer in which the junction is represented by the variable tap, an adjustment can be found to suit the abovementioned practical conditions. Provided the valve stage is reasonably linear the adjustment can be carried out during signaling with equal ratio pulses, since under these conditions the value of the anode current should remain unaltered from the steady value. Alternatively the adjustment can be made by measuring the signal output of the relay contact, and can be additionally used to effect some compensation for the change of transit time of the relay when the input wave forms with time have the worst slope.

Fig. 4 shows an alternative circuit, in which the components of the valve-relay stage remain as before and perform the same functions. The received current ii now passes through only one resistance r, and shunted across 1 is the circuit comprising C, R and MB. The grid lead is connected to a tap on R, which may conveniently be a potentiometer. It will beapparent that C will charge to a voltage equal to 11.1, less the forward voltage drop on MR. As and when il disappears, C will discharge slowly via R, since the discharge path via 1' and MR is blocked by the latter having a reverse voltage on it. Hence the voltage on R is a bias, of value nominally equal to the maximum value of the input signal, and opposing the latter with respect to the grid circuit of the valve. As the tap on R is moved downwards from the top, a bias is added to the input so that the zero grid circuit voltage datum line can be altered at will. At the nominal half setting, the input to the valve therefore consists of equal positive and negative excursions of the signal waveform, i. e., double current signals. At the literal half settin the positive excursion exceeds the correct amount by one half the forward voltage drop on MR, and the negative excursion is deficient by the same amount. If this effect can not be ignored in comparison with the input signal voltage, then it can be mitigated by biassing the valve by the same amount, since the voltage drop on. the rectifier particularly at low levels is a form of contact potential and is constant. The conversion circuits of. Figs. 3 and 4 are therefore amenable in all respects to the use of commercial components, since the time constant of CR need only be a minimum of say 20 times the longest signal cessation period, whilst converting 1'! plus T2 in Fig. 3, or R in Fig. 4, into potentiometers gives a ready mean of adjustrnent in situ. The use of this adjustment for other purposes will be discussed later; at this stage it is only necessary to point out that it can be used to shift the input waveform from one having a polarity entirely of one sense into one whose polarity is entirely of the reverse sense. The conversion circuit can be used for other purposes where it is desired to get rid of, or alter at will, the D. C. component of an input waveform, for instance in depicting waveforms by means of as oscillograph without using internal shifting means.

One of the practical advantages of Fig. 4 over 3, lies in the fact that grid current passed by V when the positive input exceeds a certain amount, does not affect the voltage on C, and hence no grid current limiting resistance, such as Hg in Fig. 3, is needed in Fig. 4, although one has been shown. For when the current is in the direction shown, conductin and grid current drawn via the tap on R only alters the voltages on the separate portions of B. When the input disappears slowly grid current then acts as a load on C until the grid circuit input voltage has fallen sufficiently, but provided the tap is not far moved from the centre position the resultant change in the rate of decay of the voltage on C will not be serious. A similar effect will be apparent if the input is of reversed sign, and MB is correspondingly reversed, but in this case the grid current load will occur during the signal cessation period.

In applying these forms of the invention to an incoming rela' set in an automatic telephone exchange, the resistances Ti, T2 or R may take the place of one coil of the normal A relay, the other coi being replaced by a similar total resistance in the other line wire. The normal local battery may then be used for energising the valve, the line current being fed out via the resistance.

In Fig. 5 which shows the simplest application using the circuit of 4 with the same lettering, received current is new that present in the line vire 15/ i, which in the absence of tn currents is the same as that in LWZZ. This rent may be produced from or conjunction with, the local battery E the distant trans mission arrangements. The circuit shows line current feed via a speech transmission. bridge L4, L4, and K as for 2. A corresponding resistance r is inserted in the remaining line wire for the purpose of maintaining a balance with respect to earth. Since as regards the line current, the termination is resistive, advantage can also be taken of the current gain in the valve circuit to apply any of the well-known means for improving the received waveform of the signals. This can be done either on the line side and treating the resistances r as the load, or in the anode circuit of the valve, provided the latter is linear over the significant portion of the double current input waveform resulting from the conversion circuit.

If the signal transmission system is such that there is the possibility of earth currents, then the above simple application must be amplified in order to nullify this effect. This can be done by using the voltage on the remaining resistance r in the other line, to operate a similar conversion and. valve circuit. Since the desired line current will produce a voltage of opposite polarity with respect to earth. the rectifier MB in the second circuit will have to be reversed and the changes of anode current will be of opposite sense. These can be utilised by passing them through the sec ond winding of the polarised relay formerly used for neutralising the steady anode current. The need for such a neutralising windingthen disappears. A source of anode potential which is positive to earth, will be needed forthe second valve. Although this method nullifies the effects of earth current since the input voltages produced thereby are of the same polarity the presence of earth current inputs is liable to cause non-linear operation of the valves during signaling and at the signal magnitude insufficient to have operated the relay, thereby defeating the purpose of the invention.

Fig. 6 shows a circuit arrangement in which the input to the conversion circuit and valve is unaffected by earth currents, and also uses only one valve. The line resistances r, of equal value, are connecteddirectly to earth and battery. The conversion and valve circuit of Fig. 3 is connected on the line side of these resistances, except that MB. is reversed and TI, T2 is fed from the positive line by a resistance T3. The action is the same as before, eXcept that there is now a permanent positive bias on the grid of the valve, and the eifect of the flow of line current is to produce negative input signal (hence the reversal of MR). Earth currents passing through the line resistances, now produce no change in the voltage across r3, r2, Ti and hence no effect on the valve. The conversion circuit of Fig. 4 cannot be used properly here, as the permanent positive input biases the rectifier so as to be non-effective during the cessation of the input signal; the presence of the series condenser in Fig. 3 prevents this effect. Hence the output of the conversion circuit to the change in line currents is precisely the same as before, plus the constant positive input bias. The effect of this constant bias may be mitigated by increasing the value of Re, above the value normally used, with consequent improvement of the linearity of the valve stage by virtue of the cathode feedback.

The effect of the anode current also flowing through the line resistances is not found to be deleterious, if Re is made much larger than 1' and the line resistance is less than n, where ,u is the lies in the fact that the fractional voltage on rl must be less than on 12, by v y if absolute equality of anode current excursions is desired. However, this is easily arranged, particularly if a is large, by using TI and 12 as a potentiometer.

Although signaling employing non-symmetrical wavefronts cannot give as good results as when symmetrical wavefronts are present, since the minimum pulse time for no mutual interference is less, it is nevertheless frequently encountered. Provided such a system is otherwise satisfactory in its limited performance as regards mutual interference, the invention can be used to give an operating waveform passing through zero amplitude at, the correct time interval corresponding to the input pulse. By this means distortionless operation can be secured at any input level, provided the difference in waveform with time of the wavefronts remains constant.

A typical instance is the normal transmission of pulses over telephone systems by means of successive loops and disconnections initiated by the sending contact. The battery and responsive relay are both located at the receiving end, and as is well-known the disconnection waveform is much more prolonged than the loop. If the transmitting medium remains otherwise linear, then the non-symmetry is due entirely to the non-linearity introduced by the contact, and the waveforms although different are consistently repeated and independent of the current magnitude. A fractional value can therefore be found where the input time interval is reproduced, and this value will be of course larger than 0.5 in this case. Hence by adjusting the invention so as to add the appropriately larger bias, on a fractional basis, distortionless reproduction can be obtained independent of received level. This adjustment can be carried out simply by means of the potentiometer before referred to (i. e., H and r2) as a potentiometer in Fig. 3, or R in Fig. 4.

In the case of transmission by loops and disconnections over a cable pair having primarily distributed capacity and resistance, the required fraction of the input signal at which the correct time interval is present, is 0.75. Hence the bias effect would be adjusted to have a magnitude 0.75 that of the signal input. It will be seen that the difference between this adjustment and that for symmetrical waveform (i. e., 0.5) is not so great as to give any trouble, and apart from the overriding consideration of minimum pulse time is well worth consideration, particularly in the transition period between the conversion of the loop/ disconnection method to a linear transmission method.

Other forms of non-symmetry will doubtless occur to those skilled in the art, where the cure is otherwise too expensive, such as the tail pro duced on pulses of frequency when transmitted over a line or via certain classes of resonant circuit networks. 7

Although stress has been laid in the specification on the use of the invention to produce double current waveforms in order that a polarised relay may be used to the best advantage, on the asumption that with symmetrical Waveforms and an attenuated signal this method gives the least distortion, it does not follow that this use is the most economic if either of these assumptions is not met. At least one system having non-symmetrical waveforms can be'worked with the invention as described above, and the necessity of introducing a valve at once raises the question as to whether the attenuation of the signal now means anything. It hasbeen shown that the in-- vention can be adjusted to give in effect a definite indication of the transmitted pulse time and there is no reason why the action of the valve stage should not be made non-linear with a view to regeneration of the transmitted rectilinear wavefront, other than considerations of signal to noise ratio in the received signals. The relays could then be of non-polarised type, the distortion would not be seriously greater, and the termination would be much cheaper.

For instance, the valve V could be arranged so that only positive or negative grid voltages produced a change of anode current, by means of a cut-off bias in the first case or by zero bias in the second. If the grid base of the valve is then small compared with the magnitude or the effective polarity of the converted signal and the anode current change for this grid base is large enough,-even after the valve has aged, adequately to operate the relay, then the anode current change will be substantially rectilinear and of the correct time duration. The non-polarised relay is then working in a local circuit, with constant current amplitude as if from a local contact, so that impulse distortion will be a minimum, particularly if a high speed type of relay is used. These conditions are not very ono-rous with modern valves, since although the maximum anode current change may be limited if the grid base is kept small, the anode voltage swing can be increased at will by using a pentode with a higher anode voltage than screen voltage. Alternatively, and for certain cases, a simple D. C. amplifier stage preceding valve V may be used.

As remarked before the only limitation to this method is the possibility of interference by means of received noise. But the effects of such noise are no worse than for the former case using a linear valve stage and polarised relay, since noise which occurs at around the zero transit time of the double current signal will equally affect the polarised relay, .and noise elsewhere during the signal will again affect both to the same degree, if both valve/relay circuits have comparable input voltage/operate current sensitivities. The non-linear valve stage is in fact a limiter and as such the output can only be affected by input voltages within the non-limiting range. Use of a non-linear valve stage as indicated, does not of course affect the adjustment of the invention for non-symmetrical waveforms, or any well-known methods of waveform improvement prior to the conversion circuit.

The invention can be used with single current signals which do not attain zero amplitude, Two cases occur, one in which the single current signal is biased by a constant amount, and the other in which it is biased by an amount proportional to the single current change.

The first case has already been demonstrated in Fig. 6, in which a local bias is rendered ineffective by using the conversion circuit of Fig. 3. The bias provided by C on reduction of the input (1. e., the single current signal effect) is merely of value equal to this reduction, and as the signal input from T2 also reduces by this amount, then the adjustment of the circuit is precisel the same as before; the constant bias being taken account of in any well known manner, The circuit of Fig. 3 also functions to a signal which increases over the signal period, by reversing the rectifier, as was demonstrated in Fig. 6. The circuit of Fig. (i will also function in the presence of a constant bias, except that it will not act properly to the first signal if the latter is of increasing magni- 10 tude. This is because the working bias on C is always equal to the maximum signal input.

The second case in which the signal is itself biased, but by an amount proportional to the change denoting the signal, can also be utilised by both circuits. The same proviso holds namely that 4 will not act properly to the first signal if the latter be an increase in magnitude. In general if it be assumed that the signal is a positive effect and is biased proportionately in a positive manner, then since both circuits are capable of providing a completely negative output by sliding the potentiometers downwards to the limit, it follows that there is always some intermediate position Where the output can be made to pass through zero at time intervals equal to the transmited signal. This position is easily calculable with knowledge of the proportional bias on the signal, For a negative signal with a negative proportional bias the rectifiers are reversed.

There is thus no limit to which the signal may be proportionately biased, yet which will not permit the conversion circuit to deliver the signal waveform passing through zero at the correct time intervals and hence to reproduce the signal faithfully. The only limiting factor in such a signaling system is the magnitude of the signal change itself at the reception apparatus, For instance suppose it is desired to signal by slightly altering the amplitude of an otherwise large effect, say, D. C. or A. '0. power transmission. If equal positive and negative outputs are required from the conversion circuit, then the potentiometers in either circuit must be moved until the bias effect equals the mean of the two signal conditions, on a percentage basis. The output from the conversion circuit will then consist of equal positive and negative excursions, and of total amplitude equal to the received change only; the mean of the two signal values being completely eliminated. Of course, the potentiometer can still be adjusted to give non-equal positive and negative excursions, if this should be needed, for instance owing to the non-symmetry of the waveforms.

The transmitted single current effect may therefore consist of mere changes in amplitude, on a fractional basis, and provided the fraction is known the invention can be easily adjusted to reproduce faithfully the transmitted time, This facility opens up very important and novel methods of selective signaling using only one type of signal effect, and certain embodiments will now be discussed.

Selective signaling may, for instance, be effected by means of varying amplitudes of one type of signal effect.

Any single current signal effect can be used, as before, and it will be assumed that it is converted to the corresponding D. C. form, although this is not necessary for the correct action of the conversion circuit. It will also be assumed that the transmission system is linear, insofar as a given percentage change at the sending end is faithfully reproduced at the receiving end, ignoring any non-symmetrical transient effects which can be treated-on their own merits (or de-merits) as discussed heretofore.

Let the received signal effect amplitude prior to signaling be A; in general this will be the maximum received amplitude. Consider the transmission of a signal by means of a reduction of amplitude amounting to 2 1.11, where p is a fraction less than unity. Then as before if it is wished that the conversion circuit give equal positive and negative excursions. the bias effect will be adjusted so as to be equal to (1p)A, i. e., the mean of the two signal amplitudes. Now consider the action of another conversion circuit to this signal, but let the second circuit be adjusted to respond normally to a reduction of signal amplitude of 2q.A. The bias efiect on the latter will have been adjusted to (1-q) A, which means that until the reduction of the signal amplitude is qA, the output from the secondconversion circuit will not change in sign, and hence will not properly operate the succeeding valve stage, etc. Hence if 250 is less than q, only the first circuit will operate, and if greater then both will operate. Thus by splitting up the available amplitude A into n equal parts, arranging that signals consist of reductions in amplitude which are an integral number of such parts, adjusting the n receivers in their conversion circuits to produce biases equal to the means of the 12 changes in amplitude, then the receivers may be successively and simultaneously operated from I to n. In this manner and provided the amplitude is sufficient to operate one receiver, over the maximum transmission attenuation, selective signaling with the facility of holding operated those receivers of higher bias adjustment, may be readily accomplished, using only one type of signal and independently of received level. Where the receiver biases are adjusted to give non-equal excursions of output in order to mitigate the effects of non-symmetry in the received waveforms, all the signals will have this effect, and the slight modification to the above procedure will b apparent.

This method of selective signaling can also be used to discriminate powerfully against other unwanted signals such as noise or speech. In this case the signals would consist of alternating current whose amplitude is reduced by precisely known fractional amounts to represent various signals. The signal receivers must remain on the line for the purpose of operating to supervisory signals subsequent to speech, and will be affected by the varying speech levels. But only rarely will the positive and negative excursions produced by the conversion circuit from speech be in precis'ely the correct ratio so as to correspond to a genuine signal, and then only for very short time periods. Consequently it is only necessary to arrange a subsidiary circuit to continuously compare these speech signal excursions, and to arrange that when the right ratio is not present that a blocking effect is obtained to nullify the operations of the receiver proper. Such could consist for instance of two series rectifier/condenser circuits connected in parallel and to the positive and negative output excursions from the conversion circuit. The rectifier would be connected so that one condenser obtained charges proportional to the positive and the other charges proportional to the negative excursions. A potentiometer connected between the ends of the condensers not joined together, would be set at the correct ratio and the voltage between the arm and the condenser join used to energise the blocking apparatus via a valve. A voltage would be present for all signals not having the correct ratio, whilst during idle periods or at the cessation of speech, the output would always tend to zero'by leakage of the condenser charges, so that subsequent signaling can take place.

The above forms only one .of the many ways in which this method of selective signaling may be utilised to the full, and it will be noticed that selection on a basis of frequency, even for speech immunity, is not entirely necessary. 7

The invention may be employed to enable long period signals to be separated from impulse signals.

This facility would enable a considerable improvement in performance and simplification in design of telephone apparatus, particularly in the repeating relay sets and selectors. With the present system of signaling in which only two line current conditions are recognisable in a given direction, namely, on and off, the one receiving relay has to give both long period and impulse signals, for instance the seizure and clear signals, and impulsing. Since the latter signal is merely a rapidly repeated version of the former signals in succession, it follows that the relays associated with the seizure and clear signals have to be slow releasing, and also not responsive to impulsing. It is well-known that they are responsive to impulsing and moreover on an input make percentage basis, and that this fault is responsible for a large percentage of the so-called impulsive failures. The only other part of the system which can break down on impulsing is the switch magnets, and each breakdown is on a time basis and does not therefore impose such limitations as the make percentage breakdown. The invention, used with or without the selective signaling method outlined above, is admirably adapted for complete separation of these long and short period signals.

To take the simplest example when used with selective signals, it would be arranged that the transmitting contacts comprised a series connection of two contacts, one for seizure and clear and the other for impulsing, in a similar manner to the switch hook and dial contacts of a subscribers installation. At the reception apparatus two responding devices would obtain, one responsive only to the presence or complete absence of the received single current effect (for instance a simple non-polarised relay), and the other responsive only to the given fractional change in the received single current effect (i. e., the invention). The sending impulse contact would transmit the given fractional change and it would be received only by the apparatus according to the invention, provided it is arranged that the residual single current effect is sufficient to hold the pure single current responsive device. Hence the contacts of the latter could be devoted solely to the seizure and clear signals and to the corresponding relay set or selector operations, without any trouble due to the impulsive regime. The contacts of the impulse responsive apparatus then merely have to energise the stepping mechanism or repeat the impulses over the next link. In this manner failure of the well-known B relay on impulsing can be avoided at all repeating stages. With regard to that function of the wellknown C relay which is concerned with its operation on the trains of impulses, it is possible to obtain an operating voltage from the conversion circuit which only exists whilst impulsing is in progress without alteration or variation of the normal function of thi circuit. Such a voltage occurs across the rectifier (i. e., in the backward direction) and consists of a series of negative voltage pulses of number and duration equal to the impulses andof magnitude equal to the change in the signal independent of the setting of the po- 13 tentiomete'r for operation of the impulsing circuit, but without the positive and permanent component. Applying such pulses to another valve stage will produce an average current value for the duration of an impulse train, which can be used to operate the C relay. The operation of the latter will still be on a make percentage basis, but this can be avoided by a simple non-linear charge and discharge circuit interposed between the negative voltage on the rectifier and the grid of the C relay valve, so that the buildup of anode current is much slower than the decay. This latter arrangement can also impart any desired degree of slow release to the C relay, the contacts of which will, of course, operate in the reverse manner to normal.

A simpler method for operating the C relay in accordance with the above requirements, using either of the conversion circuits, now presents itself, and with it the elimination of all troubles due to make percentage on both B and C relays, without requiring the above postulated selective signaling arrangement or the additional single current relay for indicating the seizure and clearing signals. This design sequence, to be described hereinafter also results in a design of relay set and selector which can work in with the existing system until such time as the latter, as regards the internal exchange system, is altered.

The operation of a device such as a C relay in automatic telephony, over a train of impulses soleiy from an impulsing contact without the aid or intervention of a subsidiary relay will now be described.

Fig. 7 shows a circuit comprising part of Fig. 4-. with however a ,pentode valve for V to operate the receiving impulsing relay P at the correct time displacements by adjustment of the tap on R, the valvefunctioning as a linear amplifier for positive and negative inputs if R is so chosen, or as a non-linear switch, it Re has a biassing current passed through it or is replaced by a bias battery so that the anode current is normally zero. The rectifier MR of Fig. 4 is now provided by the grid/cathode path of another pentode valve Vc, so that the receiving impulsing circuit above the negative battery busbar functions as before. The train-operated relay CR is connected in the anode circuit of V0 and is normally operated so that its contacts will have the reverse action to normal.

The incidence of received line current i! by the seizure signal, does not aifect relay CR since grid current is already flowing, but when impulsing occurs by a reduction in ii, a negative impulse voltage appears acros the grid/cathode of V0 which reduces the anode current through CR to zero to cause its release. Now a pentode valve exhibits high dynamic impedance to any positive anode voltage change, and since CR is inductive its back E. M. F. when the current falls makes the anode positive. Hence the reduction of anode current follows substantially the change of grid voltage. Upon restoration of the original value of ii at the end of the impulse, the negative grid/cathode voltage on V0 disappears and grid current is restored. Vc becomes conducting again and the anode current commences to rise. The back E. M. F. on CR now opposes E. but cannot exceed it, so that the anode voltage is depressed below its former steady state value. If the latter steady state anode voltage i chosen so that it is not greater than the knee value of the anode characteristic of V0, then the impedance of the anode is very low for anode voltages less than the knee value. Hence when grid current is restored the rise of anode current in OR is slow and dependent on the time constant comprised of the inductance of CR and the total circuit resistance.

Now the knee voltage of a pentode is about one fifth the screen voltage, hence even if the latter is the same as the anode battery voltage, the additional circuit resistance due to the valve in the conducting sense is only 20% of the relay resistance. With lower screen voltages it is correspondingly lower. The rise of current in CR will therefore be governed almost entirely by the time constant of the relay, and the operation of CR will be slow. In this manner it can easily be arranged that the operate lag of CR exceeds the maximum time between impulses so that CR remains released during the train of impulses. At the end of the train, the inter-train time will be sufficient to permit the re-operation of CR after the design operate lag. It will be appreciated that the time constant of the relay circuit may b modified at will, for instance increased by series inductance. It will also be seen that the re-operation of CR is dependent entirely on the inter-impulse time and not on the input make ercentage, since the valve is nonlinear and permits the anode current to fall as fast as the applied grid waveform.

The circuit as described up to the present suffers from one disability, namely, that th reoperation of CR after a clear signal is unduly delayed. When 21 falls to zero on the clear, CE is again released, which is normal in relay sets, but it will only reoperate after the' voltage on the condenser CR has fallen substantially to zero. Owing to the requirement that the time constant CR shall be large for correct operation of the impulsive portion of the circuit, 'it follows that the total decay time of the voltage on the grid of Va is much too long. Moreover it depends on the magnitude of the received voltage on r compared to the cut-off bias of V0. Means will now be described for overcoming these defects in the present circuit, but in general they are more easily overcome by duplicating the conversion circuit for the CR relay circuit, and operating it from the local battery via the repeating contact of P. The latter will be described later.

If it is desired to use the received voltage to operate the CR relay circuit and also to overcome the above disabilities, a form of limiting must first be applied to the voltage actuating Vc. Circuit elements mi, MR2 and bias e can perform this function (shown in dotted lines in Fig. '7). MR5, whilst permitting the normal flow of charging current for condenser C via the grid/cathode of V0, absorbs the negative input to the latter less an amount 6, when 2'! is reduced for an impulse. MR2 is conducting on this latter regime, but prevents e from appearing across the grid/cathode during the former regime. The negative input to V0 cannot therefore exceed e for any input current ii, is constant for such time as the condenser C voltage exceeds e, whilst the normal discharge of the condenser remains unaffected. Provided this negativ input 6 exceeds that required to cut off the valve, the action of the latter and relay CR will not be altered during a train of impulses. It is now necessary to arrange an over-riding control of the rise of anode current on the clearing signal. This is performed by the circuit K, S in the screen lead of V0. Prior to the receipt of the negative grid input during impulsing, it is arranged that 15 the screen voltage shall be low'by the flow of screen current through S. As soon as the negative input e is received to stop the flow of cathode current, the screen voltage rises and with it the value of the negative grid input. to stop cathode current. Hence after a time depending on the choice of KR, .cathode current commences to flow again despite the continued presence of e, since the latter is now insufficient. The flow of screen current reduces the time constant of the screen circuit and it is soon stabilised, whilst the anode current thereafter also attains its steady value. It can be arranged that the value of the latter under these conditions is the same as formerly, so that the circuit is ready for another seizure. The time after which the rise of screen voltage causes the cathode current to flow again, is made not less than the maximum impulse time so that the action of the circuit over an impulse (i. a, cessation of signal) is unimpaired, whilst the incidence of the original value of 2! after an impulse causes grid current to flow and rapidly restores the screen voltage to its original-value.

An alternative method of connection of the above CR relay circuit, which avoids the disabilities before-mentioned, and shows how this general method of operating the CR relay can be made to eliminate B relay failures on impulsing, and in some cases to dispense with the B relay, and will now be described.

In Fig. 8 is shown the relevant portion of the local circuit of a final selector; by deletion of the-superfluous portions as will be indicated later it serves also for a group selector or relay set. A is the local repeating contact for the incoming current pulses and may be operated in any wellknown manner therefrom or by using the invention as before described (i. e., where A is the contact of relay P in Figs. 3 to 7). The CR relay circuit to the right of the dotted line is joined now to the A make contact, so that the positive single current signal due to the current flowing through 1' to the battery E is now of constant magnitude. In this manner the negative input to V when A releases and derived from the charge on the condenser Q, is also of constant magnitude with time. The limitation efiect is now not required and. furthermore the time constant CR can now be made only just long enough to ensure that CR is held released during the break period of A. In this manner and if the remaining portions of the circuit be ignored, as for a relay set. the delay period after clearing is made a minimum. It should be mentioned that a resistance Z has been shown as an optional component, which may be necessary to limit the maximum back E. M. F. of CR to a value which th valve will stand when the anode current falls; it may also be fitted in Fig. '7 for the same reason.

In Fig. 7 and in Fig. 8, so far as the latter has been described, We have a CR relay function which is only permanent as long as the A contact is impulsing. For'instance CR cannot be held released by the release of A as might be possible in the normal circuit. Hence it is legitimate to use a CR contact to hold any other relay also associated with the A make contact which would otherwise be afiected by the impulsing regime, and in particular the B relay. For instance, a B relay is shown connected to the A make via a rectifier MRB and is held by C3 during the break period of A. (Note, the CR contacts in Fig. 8, are shown in the condition consequent upon the operation of CR by the valve Vc and battery E.) This is the normal condition prior to use of the circuit. .B will now release after CR when A is released permanently, and there can be no trouble with B on the score of impulsing. Since CR is also unaffected by the make peroentage of A and only by the break time, it can be said that this design eliminates all troubles due to holding relays over impulsing. B has been shown as a slow release type, but this is not now necessary for impulsing although it may be necessary for other reasons, whilst MRB is only necessary if the A make contact has to impulse a repeating relay such as AA as is often the case in repeating relay sets. A typical use for the B relay is shown at contact B! which places and maintains the guarding earth on the private wire during the whole time the circuit is in use, and for an extended period after CR reoperates, if it is slow releasing.

The remaining portions of Fig. 8 are concerned with the application to group and final selectors. The diagram shows the latter case but th former can be arrived at by deletin the off-normal rotary contact NR, the rotary magnet Rm, and shorting the break contacts of El, E2. The modifications are necessary because it is required that the CR relay shall function immediately the circuit is seized, in order that the vertical or rotary magnets V117. and Rm shall have the full energising period on the first break, and in order that the CR relay shall be prevented from functioning subsequently to the impulse train or trains. The latter is required in order that the magnets shall not be reseized on the clear signal. The action of the CR relay must therefore be distinctive to the first operation in a train of impulses of the A contact, but not thereafter. Off-normal contacts NV and NR in conjunction with the A break perform this distinction for the vertical and rotary trains respectively. On the first seizure, the A break removes the earth from the anode circuit of V0 and releases CR. CR! removes a short from the grid/cathode of V0 and permits the normal function of the CR relay circuit during the impulse train. At the end of the train, however, CR1 prevents the subsequent release of CR by A because NV has operated on the first vertical step. Contact CR2 permits the stepping of the magnets during a train (i. e., When CR is released thereby). After the end of the first train CR re-operates and causes the well-known relay E to operate (not shown), E2 releases CR again, thereby performing a like function to the first operation of A, whilst El changes the magnet circuit over to the rotary magnet. The circuit then functions as before to the second train, NR replacing NV. After the end of the second train CR re-operates and this circuit together with the magnet circuit restores to normal, so that the Whole can be cleared down after the call without difliculty.

There is no necessity for a B make contact in series with CR2 as is usual, because even if the A relay is released before the first train of impulses, CR cannot be held permanently released by the restoration of A, and CR2 therefore performs the ultimate release of the vertical magnet,

It should be noted that although the conversion circuit of Fig. 4 has been consistently used in the previous three sections, that of Fig. 3 can be just as well used, provided a series grid resistance to V0 is provided to mitigate the eifect of grid current on the condenser voltage.

The invention can be used to receive alternating signals; for this purpose the conversion circuits of Figs. 3 and 4 can be used without modification on alternating or rectified inputs, without smoothing, and with the same object in view, namely, operation of the succeeding apparatus at the correct time intervals on the input envelope waveform independent of the received level. Furthermore the envelope waveforms can be of known non-symmetrical type, and the signals can be constituted by known fractional changes in the received amplitude as described before for D. C. inputs.

In this case the condenser C charges up to the peak value of the input, hence the peak value takes the place of the maximum D. 0. value in the adjustments. The output is of course alternating (or pulsatin if rectified) with a superimposed bias, so that if subsequent D. C. operation is required the valve V will be arranged to be operative only to the positive caps whose magnitude is determined by the degree of bias necessary for correct impulsing on the received signal envelope. Smoothing subsequent to the conversion circuit need not affect the distortion greatly, if it is linear and attains steady stat during signaling. In the case of Fig. 4 and when fed with alternating input, the resistance r can be formed by the A. C. source impedance.

As the voltage on the rectifier MR will now always have a negative pulsating component both prior to and durin signaling, it cannot be used directly for operation of a train-operated relay as shown in Fig. 7, but the locally operated circuit of Fig. 8 can always be used.

When fed with alternating or rectified input, the conversion circuit can also be used for reproducing only the top caps of the input waveform, and the cap can be varied at will from the full half cycle to nil merely by varying the bias obtained from the potentiometer R. The ratio of the cap to the half cycle remains independent of input level, for a given setting of R. One use for this efiect is the production of a constant alternating output by subsequent "chopping of this cap by means of a limiter. Adjustment of the angular displacement of the top cap can then be used to ensure that a given harmonic in the limited output wave is absent in the well-known manner.

The invention consists in essence of circuits whereby any received signal waveform or envelope with time may be converted into positive and negative excursions between successive time intervals corresponding to the time input at the transmitter.

The same circuit may be adjusted simply so as to provide the correct time intervals between points of zero amplitude on any known received signal waveformwhether it have symmetrical or non-symmetrical \vaveironts, and independently of received level. The signal may also consist of known fractional changes of a given transmitted effect.

When using valve operated relays or other devices, the circuits of the invention are extremely simple, capable of beingcomprised by commercial components, and susceptible to design so that the eilects of unwanted characteristics of the components may be rendered negligible. The valve can be used either as a linear device, with the aid or" feedback in any well-known manner, or in a radically non-linear manner which is also 18 susceptible to design by well-known and tried methods.

A primary function or the invention is to enable distortionless reproduction of transmitted time signals over a medium where substantially steady state reception is attained, although the wavefronts with time may be highly distorted, non-symmetrical and vary with received level.

The preferred method of operation of the invention is obtained by prior transmission of the maximum signal level which corresponds to many present signaling systems, and in particular to automatic telephony. The invention can be used without prior transmission of the maximum signal level, provided some distortion of the first signal is tolerable or alternatively the prior sig nal level is a known fraction of the subsequent maximum signal level.

The invention can be used on received wavefronts of known non-symmetry and needs no circuit change to operate on symmetrical wavefronts. It can therefore be used on the standard loop/disconnection signaling methods in automatic telephony in order to give an immediate improvement to impulsing, and only needs a single readjustment to operate on symmetrical wavefronts provided by a subsequent improvement in the transmission system, which has in view the operation to a shorter impulse time or over longer cable circuits.

The valve operated relay need not be of polarised type, without any great loss in repetition accuracy,

Since the input to the valve/relay stage consists of positive and negative excursions, whatever the type of signal transmitted, the relay is not susceptible to oscillations on the received waveform, unless these pass through values equal to those between which the desired time interval is attained. This limitation cannot be overcome with any other type of reception apparatus, since such effects are indistinguishable from the true signals.

The circuits of the invention can be used for distortionless reproduction of transmitted signals which consist only of known fractional alterations or" a given effect. In this manner selective signaling using only one efiect can be arranged.

The circuits of the invention enable the complete separation of the impulse time signals from that portion of the transmitted efiect which persists over a long time and which may be likened to a carrier for the signals. In this manner the operation to the carrier may be divorced from and uniniiuenced by the impulse signals. As a typical example the function of the B and C relays in automatic telephony may be rendered independent of the make percentage of the impulse signals.

The invention may be used on alternating or direct current signals without modification, and with the same objects in view as cited above. It may also be used for waveform analysis or modification (for instance to feed a limiter stage for providing waveforms of given type).

I claim:

1. In a signaling system in which current initially flows to a receiving equipment and in 70 which signals are transmitted by periodically in terrupting said current, means for deriving a biasing effect from said initial current which is proportional to the value thereof and which remains substantially constant during said inter- 75 ruptions therein, and signal receiving means means to be energized to substantially equal eX- .tents in positive and negative senses during signal transmission.

3. In a signaling system in which the current by which signals are transmitted varies from one value to another, signal receiving means, and means for deriving a substantially constant biasing effect, which is equivalent to the mean of said two values, from the signaling current, said signal receiving means being adapted to be jointly controlled by the signaling current and said biasing effect so as to be energized in substantially equal but opposite directions during signal transmission.

4. A signaling system as claimed in claim 3 in which the means for deriving said biasing effect comprises an aperiodic circuit including an asymmetrical conductor so as to provide one time function therein during the portion of a signal ii pulse when the biasing efiect is created and a different time function during the other portion of the signal impulse.

5. A signal receiving system comprising a thermionic valve having at least a cathode, control grid, and an anode, a relay having a winding serially connected in the cathode to anode path of said valve, a network consisting solely of resistance, capacitance, and asymmetrical conductance elements connected between the control grid and cathode of said valve, and a signal current path including a portion of said network, said network being arranged to apply an alternating signal voltage to said control grid when uni-directional pulsating current flows in sai' signal current path.

6. In a signal receiving system, a path over which signaling currents are at times transmitted, a resistor connected in said path, a condenser and a rectifier serially connected across at least a portion of said resistor, a discharge path for said condenser, a signal receiving relay, and means responsive to the voltages developed across said resistor and said condenser by the signaling currents flowing in said path for operating said signal receiving relay accordingly.

BERTRAM MORTON HADFIELD. 

